Force has magnitude and direction, i.e., it is a vector. Types of devices which are currently used to measure force include accelerometers utilizing capacitive, piezoelectric, and piezoresistive silicon structures. Such devices usually measure only a force magnitude, or perhaps only one single force component of a multiple component force vector. Such devices commonly use capacitive sensing or piezoresistive sensing.
Capacitive Sensing: Capacitive sensing can be used to measure force by detecting displacement. For example, movement of a first conducting surface with respect to a second stationary and parallel conducting surface results in a change in the capacitance between the conductors. The displacing force could cause a change in the spatial distance separating the two conducting surfaces or a change in the overlapping area of the conducting surfaces. A capacitance change therefore would be directly related to applied force or acceleration. Thus, a measurement of capacitance change results in a measurement of the force magnitude which caused the displacement of the conductor. Such structures as found in the current art often either do not discriminate with respect to force direction or, if they do, they generally discriminate in only a single direction. An example of such a device is described in Rudolf, U.S. Pat. No. 4,483,194 (incorporation by reference is intended), which describes a hinged cantilever structure which uses capacitance sensing for an accelerometer sensitive to a single dimension. The present invention described below provides several embodiments using capacitive sensing for the measurement of multiple components of force using a single monolithic device.
Piezoresistive Sensing: Piezoresistive sensing can be used to measure force by detecting strain induced in a beam subject to a deflecting force. Piezoresistive devices use materials with strain dependent resistivity. Compressional strain results in a resistance change of opposite sign to that of tensile strain. The dependence of resistance changes on strain or force can be very nearly linear, especially over a defined range of force. The magnitude of a resistance change is a direct measure of strain and therefore a direct measure of a strain inducing force. Well known materials which display piezoresistive properties are semiconductors and metals; both have been previously used widely in strain gauges and accelerometers. Semiconductors tend to display a larger resistance change in response to applied force than do metals. However, semiconductors also tend to be more temperature sensitive than metals. This temperature sensitivity (as well as certain non-linearities) can be compensated for using IC compensation circuitry. One temperature circuit compensation approach is described in "A Batch-Fabricated Silicon Accelerometer," by L. M. Roylance, IEEE Transactions on Electron Devices, Vol. ED-26, 1911-1917 (incorporation by reference is intended). Silicon is often the piezoresistive material of choice because of its ruggedness and because of its advanced, versatile and accessible technology which lends itself well to the batch fabrication of piezoresistive force sensors. Silicon also has the advantage of being highly resistant to mechanical fatigue. Metallic strain gauges are much more susceptible to mechanical fatigue degradation. Several embodiments of the present invention described below provide methods for the independent measurement of multiple components of force using single monolithic devices and piezoresistive sensing methods.
Accelerometers: A variety of accelerometers is disclosed in prior patents and publications. Such devices often employ capacitive, piezoresistive, or piezoelectric sensing methods by utilizing an inertial mass attached to flexible semiconductor material to detect acceleration. The majority of these accelerometers have a single axis of sensitivity, or do not discriminate force direction. Multiple axis sensitivity accelerometers can be and have been achieved by combining multiple single axis resolving devices along more than one axis within a single hybrid accelerometer package. However, this approach tends to increase the size of the accelerometer. Related assembly can be costly. In addition, so-called single axis resolving devices may be prone to errors from off-axis acceleration components. An advantage in cost reduction, size, design options, and performance can be expected if a single monolithic structure is used to measure multiple independent components of a force.
There is also a need for developing improved sensing methods. Piezoresistive sensor methods are highly temperature dependent and require special efforts to eliminate the temperature dependence. Piezoresistor sensitivities are also doping and orientation dependent. Capacitive sensing methods have been used for some accelerometers and provide attractive features. They are relatively insensitive to temperature, are not doping dependent, and are easily fabricated. Multidimensional capacitive sensing methods capable of assisting in multidimensional sensing would be useful.
The article "A Batch-Fabricated Silicon Accelerometer" (L. M. Roylance and J. B. Angell), which appeared in IEEE Trans. on Electron Devices, Volume ED-26, No. 12, December 1979, pp. 1911-1917 (incorporation by reference is intended), describes a single cantilever beam etched from a single crystalline silicon with a diffused resistor acting as the strain sensing element. This device was designed with the intent to have maximum sensitivity to linear acceleration in a single direction only, i.e., to be insensitive to simultaneous acceleration in the other principal cartesian directions.
Walker, U.S. Pat. No. 4,315,693 (incorporation by reference is intended), describes an optical sensing method for measuring angular acceleration using a passive ring Fabry-Perot interferometer. The system requires multiple lasers, and is necessarily large since three ring Fabry-Perot interferometers must be aligned along the principal axes to resolve the three components of rotation.
Sulouff et al., U.S. Pat. No. 4,522,072 (incorporation by reference is intended) describes a single mass loaded cantilever beam and claims "dual axis acceleration measurement."
Colton, U.S. Pat. No. 4,430,895 (incorporation by reference is intended) describes a central pedestal surrounded by a strain sensing membrane and states sensitivity to one and three components of linear acceleration.
There is a need for monolithic multidimensional force sensors that can be fabricated using existing technologies and that can resolve the various components of force. Accordingly, it is an object of this invention to provide an accelerometer capable of separating applied linear acceleration into its constituent components, i.e., of measuring force magnitude and direction. It is a further object of the invention to measure components of angular velocity and angular acceleration. It is a still further object of this invention to measure forces arising from gravitational, electric and magnetic fields. It is a still further object of this invention to measure acceleration or force components using capacitive or piezoresistive sensing methods, and to provide improved sensing methods.